perf(validators): reduced GC pressure

This PR is a follow-up on go-openapi#169, which cames with innocuous changes, but already
reduced the total number of allocations when validating a swagger spec.

We reduce further the pressure on go's GC by pooling temporarily
allocated objects: validators and results.

When recursively allocating validators, calls to "Validate()" redeem to the pool the allocated
validator. This means that a validator is usable only once.

A similar approach is taken for `Result`s: upon merge, the merged result
is redeemed to the pool.

Eventually, temporarily allocated `spec.Schema`s are also pooled.

In order not to break existing interfaces, this is enabled by default
only when validating a spec (`Spec()`) or when using
`AgainstSchema()`.

Another option at the spec level allows for skipping the gathering of schemas
into the result ("schemata"): these schemas must be cloned to remain
valid after their parent is redeemed to the pool.

Comment on the benchmark:

CPU profiling shows that about 30% of the CPU time is spent managing
garbage.

Memory profiling shows that our benchmark (validating the k8s API spec) goes
down from 25M allocated objects down to ~ 17M (go-openapi#169 went down from 60M to 25M).

The `Validate` method generates only ~ 5M of those, out of which
~ 4M are caused by the schema unmarshaling & expansion and 1M by string
allocations for the validator "path".
Unfortunately, classic string interning methods don't work well with this dynamic "path",
so I leave it there.

Integration tests (go-swagger/go-swagger#3064)
show a significant improvement when running go-swagger validate in codegen.

* contributes: go-swagger/go-swagger#2649

Further improvements would most likely come from optimizing the
validation of defaults and examples (which amount for 2M allocs).

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

BENCHMARK.md

@@ -2,7 +2,7 @@
 
 Validating the Kubernetes Swagger API
 
-v0.22.6: 60,000,000 allocs
+## v0.22.6: 60,000,000 allocs
 ```
 goos: linux
 goarch: amd64
@@ -11,7 +11,7 @@ cpu: AMD Ryzen 7 5800X 8-Core Processor
 Benchmark_KubernetesSpec/validating_kubernetes_API-16         	       1	8549863982 ns/op	7067424936 B/op	59583275 allocs/op
 ```
 
-After refact PR: minor but noticable improvements: 25,000,000 allocs
+## After refact PR: minor but noticable improvements: 25,000,000 allocs
 ```
 go test -bench Spec
 goos: linux
@@ -20,3 +20,12 @@ pkg: github.com/go-openapi/validate
 cpu: AMD Ryzen 7 5800X 8-Core Processor
 Benchmark_KubernetesSpec/validating_kubernetes_API-16         	       1	4064535557 ns/op	3379715592 B/op	25320330 allocs/op
 ```
+
+## After reduce GC pressure PR: 17,000,000 allocs
+```
+goos: linux
+goarch: amd64
+pkg: github.com/go-openapi/validate
+cpu: AMD Ryzen 7 5800X 8-Core Processor             
+Benchmark_KubernetesSpec/validating_kubernetes_API-16         	       1	3758414145 ns/op	2593881496 B/op	17111373 allocs/op
+```

benchmark_test.go

@@ -21,6 +21,7 @@ func Benchmark_KubernetesSpec(b *testing.B) {
 
 		for i := 0; i < b.N; i++ {
 			validator := NewSpecValidator(doc.Schema(), strfmt.Default)
+			validator.Options.SkipSchemataResult = true
 			res, _ := validator.Validate(doc)
 			if res == nil || !res.IsValid() {
 				b.FailNow()

default_validator.go

@@ -50,31 +50,25 @@ func isVisited(path string, visitedSchemas map[string]struct{}) bool {
 	}
 
 	// search for overlapping paths
-	frags := strings.Split(path, ".")
-	if len(frags) < 2 {
-		// shortcut exit on smaller paths
-		return found
-	}
-	last := len(frags) - 1
-	var currentFragStr, parent string
-	for i := range frags {
-		if i == 0 {
-			currentFragStr = frags[last]
-		} else {
-			currentFragStr = strings.Join([]string{frags[last-i], currentFragStr}, ".")
-		}
-		if i < last {
-			parent = strings.Join(frags[0:last-i], ".")
-		} else {
-			parent = ""
+	var (
+		parent string
+		suffix string
+	)
+	for i := len(path) - 2; i >= 0; i-- {
+		r := path[i]
+		if r != '.' {
+			continue
 		}
-		if strings.HasSuffix(parent, currentFragStr) {
-			found = true
-			break
+
+		parent = path[0:i]
+		suffix = path[i+1:]
+
+		if strings.HasSuffix(parent, suffix) {
+			return true
 		}
 	}
 
-	return found
+	return false
 }
 
 // beingVisited asserts a schema is being visited
@@ -89,7 +83,8 @@ func (d *defaultValidator) isVisited(path string) bool {
 
 // Validate validates the default values declared in the swagger spec
 func (d *defaultValidator) Validate() *Result {
-	errs := new(Result)
+	errs := poolOfResults.BorrowResult() // will redeem when merged
+
 	if d == nil || d.SpecValidator == nil {
 		return errs
 	}
@@ -102,7 +97,7 @@ func (d *defaultValidator) validateDefaultValueValidAgainstSchema() *Result {
 	// every default value that is specified must validate against the schema for that property
 	// headers, items, parameters, schema
 
-	res := new(Result)
+	res := poolOfResults.BorrowResult() // will redeem when merged
 	s := d.SpecValidator
 
 	for method, pathItem := range s.expandedAnalyzer().Operations() {
@@ -232,7 +227,7 @@ func (d *defaultValidator) validateDefaultValueSchemaAgainstSchema(path, in stri
 		return nil
 	}
 	d.beingVisited(path)
-	res := new(Result)
+	res := poolOfResults.BorrowResult()
 	s := d.SpecValidator
 
 	if schema.Default != nil {
@@ -278,7 +273,7 @@ func (d *defaultValidator) validateDefaultValueSchemaAgainstSchema(path, in stri
 // TODO: Temporary duplicated code. Need to refactor with examples
 
 func (d *defaultValidator) validateDefaultValueItemsAgainstSchema(path, in string, root interface{}, items *spec.Items) *Result {
-	res := new(Result)
+	res := poolOfResults.BorrowResult()
 	s := d.SpecValidator
 	if items != nil {
 		if items.Default != nil {

example_validator.go

@@ -59,7 +59,8 @@ func (ex *exampleValidator) isVisited(path string) bool {
 //   - individual property
 //   - responses
 func (ex *exampleValidator) Validate() *Result {
-	errs := new(Result)
+	errs := poolOfResults.BorrowResult()
+
 	if ex == nil || ex.SpecValidator == nil {
 		return errs
 	}
@@ -74,7 +75,7 @@ func (ex *exampleValidator) validateExampleValueValidAgainstSchema() *Result {
 	// in: schemas, properties, object, items
 	// not in: headers, parameters without schema
 
-	res := new(Result)
+	res := poolOfResults.BorrowResult()
 	s := ex.SpecValidator
 
 	for method, pathItem := range s.expandedAnalyzer().Operations() {
@@ -105,6 +106,8 @@ func (ex *exampleValidator) validateExampleValueValidAgainstSchema() *Result {
 					if red.HasErrorsOrWarnings() {
 						res.AddWarnings(exampleValueItemsDoesNotValidateMsg(param.Name, param.In))
 						res.Merge(red)
+					} else {
+						poolOfResults.RedeemResult(red)
 					}
 				}
 
@@ -114,6 +117,8 @@ func (ex *exampleValidator) validateExampleValueValidAgainstSchema() *Result {
 					if red.HasErrorsOrWarnings() {
 						res.AddWarnings(exampleValueDoesNotValidateMsg(param.Name, param.In))
 						res.Merge(red)
+					} else {
+						poolOfResults.RedeemResult(red)
 					}
 				}
 			}
@@ -220,7 +225,7 @@ func (ex *exampleValidator) validateExampleValueSchemaAgainstSchema(path, in str
 	}
 	ex.beingVisited(path)
 	s := ex.SpecValidator
-	res := new(Result)
+	res := poolOfResults.BorrowResult()
 
 	if schema.Example != nil {
 		res.MergeAsWarnings(
@@ -266,7 +271,7 @@ func (ex *exampleValidator) validateExampleValueSchemaAgainstSchema(path, in str
 //
 
 func (ex *exampleValidator) validateExampleValueItemsAgainstSchema(path, in string, root interface{}, items *spec.Items) *Result {
-	res := new(Result)
+	res := poolOfResults.BorrowResult()
 	s := ex.SpecValidator
 	if items != nil {
 		if items.Example != nil {
@@ -275,7 +280,7 @@ func (ex *exampleValidator) validateExampleValueItemsAgainstSchema(path, in stri
 			)
 		}
 		if items.Items != nil {
-			res.Merge(ex.validateExampleValueItemsAgainstSchema(path+"[0].example", in, root, items.Items)) // TODO(fred): intern string
+			res.Merge(ex.validateExampleValueItemsAgainstSchema(path+"[0].example", in, root, items.Items))
 		}
 		if _, err := compileRegexp(items.Pattern); err != nil {
 			res.AddErrors(invalidPatternInMsg(path, in, items.Pattern))